1. Field of the Invention
The present invention relates to electrostatic discharge (ESD) protection devices. In particular, the present invention relates to ESD protection devices that have islands and breakdown-enhanced layers.
2. Description of the Related Art
As products based on integrated circuitry (ICs) become more delicate, they also become more vulnerable to impact from the external environment, especially ESD stress occurring when one pin of an IC is grounded and anther pin of the IC contacts an electrostatically-precharged object. Therefore, input pins, output pins, input/output pins, and the power-bus pins for an IC for communicating with external systems must all be well equipped with ESD protection devices or circuitry to meet the minimum level of ESD robustness required by commercial applications.
NMOS (Negative-type Metal On Semiconductor Field Effect Transistor), either with the gate grounded or with the gate coupled to a positive voltage during an ESD event, have commonly been used as primary ESD protection devices for ICs. It is well known that the drain contact of an NMOS must be kept a few microns apart from the gate of the NMOS. What is implied is that the drain side of an NMOS confronting ESD stress in the front line must have a distributed resistor connected in series between the channel under the gate and a drain contacts coupled to an IC pad, and the resistance of the distributed resistor must be larger than an acceptable value. If the ESD transient current starts to localize at a weak spot near the gate, it causes the entire ESD current to rush in, thereby causing local heating and eventually damaging the NMOS. On the other hand, the distributed resistor helps to raise the potential of the adjacent diffusion area, and hence induce a more uniform ESD current flow towards the whole channel.
The advanced salicide process, which forms silicide material on drain/source regions to reduce the resistance of active regions and speeds up the circuit operation rate, however, makes construction of the above-mentioned resistor more difficult and costly.
One known solution for the problems induced by the salicide process is to use the salicide block process, which blocks the formation of silicide on certain diffusion regions. However, this solution is inefficient due to the process complexity and the extra mask required.
U.S. Pat. No. 5,721,439 (hereafter referred as '439 patent) discloses an MOS structure comprising a number of isolated islands in the drain diffusion region (as shown in FIG. 1). The ESD transient current flows around these isolated islands from the drain contacts 10, toward the drain-gate edge, thereby increasing drain resistance to improve ESD protection.
U.S. Pat. No. 5,248,892 discloses an MOS structure comprising a resistor means whose width is substantially equal to the width of the active zone, wherein the resistor means comprises a number of strips of titanium silicide overlying a resistance zone (n-well) and extending substantially parallel to each to increase drain resistance.
U.S. Pat. No. 6,046,087 discloses an ESD protection device using a second gate as silicide-blocking mask for the drain region, wherein the second gate overlies an N-well region and separates the drain of the host transistor into two portions.